GM Service Manual Online
For 1990-2009 cars only

Diagnostic Instructions

    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provide an overview of each diagnostic category.

DTC Descriptors

DTC P0117:  Engine Coolant Temperature (ECT) Sensor Circuit Low Voltage

DTC P0118:  Engine Coolant Temperature (ECT) Sensor Circuit High Voltage

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

ECT Sensor Signal

P0117

P0118

P0118

--

Low Reference

--

P0118

P0118*

--

* Internal ECM damage may occur if the circuit is shorted to B+

Typical Scan Tool Data

ECT Sensor

Circuit

Short to Ground

Open

Short to Voltage

Operating Conditions: Engine operating in Closed Loop.

Parameter Normal Range: Varies with ambient temperature.

ECT Sensor Signal

129°C (264°F)

-40°C (-40°F)

-40°C (-40°F)

Low Reference

--

-40°C (-40°F)

-40°C (-40°F) *

* Internal ECM damage may occur if the circuit is shorted to B+

Circuit Description

The engine coolant temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. The engine control module (ECM) supplies 5 volts to the ECT signal circuit and supplies a ground to the low reference circuit.

The following table illustrates the difference between temperature, resistance, and voltage:

ECT

ECT Resistance

ECT Signal Voltage

Cold

High

High

Warm

Low

Low

Conditions for Running the DTC

    • The engine is running for more than 2 minutes.
    • The DTCs run continuously when the above condition is met.

Conditions for Setting the DTC

P0117

The ECM detects that the ECT sensor is more than 149°C (300°F) for 4 seconds.

P0118

The ECM detects that the ECT sensor is less than -39°C (-38°F) for 6 seconds.

Action Taken When the DTC Sets

DTC P0117 and P0118 are Type A DTCs.

Conditions for Clearing the MIL/DTC

DTC P0117 and P0118 are Type A DTCs.

Diagnostic Aids

    • After starting the engine, the ECT sensor temperature should rise steadily, then stabilize after the thermostat opens.
    • Test the ECT sensor at various temperature levels in order to evaluate the possibility of a skewed sensor. A skewed sensor can result in a driveability condition.
    • If the vehicle has set overnight, the ECT sensor and the intake air temperature (IAT) sensor values should display within 3°C (5°F).

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

    •  Engine Control Module Connector End Views
    •  Engine Controls Connector End Views

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

    •  Scan Tool Data List
    •  Scan Tool Data Definitions
    •  Scan Tool Output Controls

Circuit/System Verification

  1. Engine running, DTC P0117 and P0118 should not set.
  2. If the vehicle passes the Circuit/System Verification Test, then operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records Data List.

Circuit/System Testing

  1. Ignition OFF, disconnect the harness connector at the engine coolant temperature (ECT) sensor.
  2. Ignition OFF, test for less than 5.0 ohms of resistance between the low reference circuit and ground.
  3. If greater than 5 ohms, test the low reference circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.
  4. Ignition ON, using a scan tool verify that the ECT sensor parameter is at -40°C (-40°F).
  5. If greater than -40°C (-40°F), test the signal circuit of the ECT sensor for a short to ground. If the circuit tests normal, replace the engine control module (ECM).

    Important: If the signal circuit of the ECT sensor is shorted to voltage the ECT sensor may be damaged.

  6. Install a 3-amp fused jumper wire between the signal circuit and the low reference circuit. Verify the ECT sensor parameter is greater than 128°C (262°F).
  7. If less than 128°C (262°F), test the signal circuit of the ECT sensor for a short to voltage or an open/high resistance. If the circuit test normal, replace the ECM.
  8. If all circuits test normal, test or replace the ECT sensor.

Component Testing

  1. Ignition OFF, disconnect the harness connector of the engine coolant temperature (ECT) sensor.
  2. Important: A thermometer can be used to the used to test the sensor off the vehicle.

  3. Test the ECT sensor by varying the sensor temperature while monitoring the sensor resistance. Compare the readings with the Temperature Versus Resistance table and verify that the resistance is within 5 percent of the specification.
  4. If the resistance vs temperature is not within 5 percent, replace the ECT sensor.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Engine Coolant Temperature Sensor Replacement
    •  Control Module References for engine control module replacement, setup, and programming